Just because that Mum at the nursery says it is a “hardy” mum, don’t you believe it. It is very difficult to get ahold of a true “hardy” Mum, these days (the hardy Mums you see may be hardy SOMEwhere but they aren’t here). That doesn’t mean you should avoid them. Just get the ones you want for the season (like you do with every other annual) and enjoy it while you have it.
Or you can look for someone with a true “hardy” mum (an old one) and get a root from them and keep it forever. You’ll have a collector’s item.